Closed loop control of the induction heating process using miniature magnetic sensors

ABSTRACT

A method and system for providing real-time, closed-loop control of the induction hardening process. A miniature magnetic sensor located near the outer surface of the workpiece measures changes in the surface magnetic field caused by changes in the magnetic properties of the workpiece as it heats up during induction heating (or cools down during quenching). A passive miniature magnetic sensor detects a distinct magnetic spike that appears when the saturation field, Bsat, of the workpiece has been exceeded. This distinct magnetic spike disappears when the workpiece&#39;s surface temperature exceeds its Curie temperature, due to the sudden decrease in its magnetic permeability. Alternatively, an active magnetic sensor can measure changes in the resonance response of the monitor coil when the excitation coil is linearly swept over 0-10 MHz, due to changes in the magnetic permeability and electrical resistivity of the workpiece as its temperature increases (or decreases).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 09/718,289, Miniature Magnetic Sensor for Real-Time Control ofthe Induction Heating Process, filed Nov. 21, 2000, by A. E. Bentley, J.B. Kelley, and F. J. Zutavern now U.S. Pat. No. 6,455,825 which isherein incorporated by reference. This application is related toapplication, “Acoustic Sensor for Real-Time Control of the InductiveHeating Process”, by Zutavern, Kelley, and Lu, Ser. No. 09/718,293,filed Nov. 21, 2000.

FEDERALLY SPONSORED RESEARCH

The United States Government has rights in this invention pursuant toDepartment of Energy Contract No. DE-AC04-94AL85000 with SandiaCorporation.

BACKGROUND OF THE INVENTION

This invention relates generally to the field of heat treatment ofmetals, and more specifically to a method and system of providingreal-time, closed-loop control of an induction heating machine by usinga miniature magnetic sensor to measure the local changes in magneticfield close to a workpiece during induction heating.

Induction heating is a well-known process for efficiently applyingenergy directly to metals and other conductive materials for heattreating, melting, welding, brazing, tempering, normalizing, aging, orpre-heating prior to hot working.

Induction heating can also be used in non-metal applications, includingadhesive bonding, graphitizing carbon, drying, curing, and superheatingglass. In induction heating, alternating electric current is passedthrough an induction heating coil that is positioned closely to aworkpiece. Where the lines of magnetic flux produced by the inductionheating coil enter the workpiece, the alternating magnetic fields inducean alternating electric potential (e.g. voltage) in the workpiece. Thealternating electric potential drives eddy currents in a thin surfacelayer. These eddy currents dissipate some of their energy within thesurface layer by resistive Joule heating losses. The depth of resistiveheating (e.g. skin depth) is inversely proportional to the square rootof the product of three parameters: applied induction frequency,magnetic permeability, and electrical conductivity. The resultanttemperature rise in the resistively heated surface layer is related tothe specific heat, density, thermal conductivity, power level, andduration of heating. Magnetic coupling of the induction heating coil tothe workpiece depends strongly on the geometrical arrangement, amongother properties.

A common use of induction heating is case hardening of medium-carbonsteel parts, such as gears, axles, and driveshafts. Many industrialapplications require a steel part having a hardened outer surface (e.g.“case”) and an interior region of higher toughness to provide improvedstrength, wear resistance, fatigue life, and toughness. Otherapplications include induction hardening of crankshafts, valve seats,railroad rails, rolling-mill rolls, and hand tools. Induction heatingrapidly heats the outer surface layer of the steel workpiece in a shortperiod of time (e.g. 5 seconds). Above a critical transition temperature(about 760 C for 1050M steel with 0.45% C) the initial ferrite-pearlitemicrostructure (BCC) transforms into the austenite phase (FCC).

Upon continued heating of the part, the transformed austenite layerthickens and extends deeper from the surface. Optimum peak surfacetemperatures can be 870-925 C, depending on the carbon concentration,and the desired depth of hardening. For some applications, the peaksurface temperature can be as high as 1200° C. Final hardening of theouter layer occurs when the heating power is shut off and the part isquenched (e.g. rapidly cooled from the outside to less than 200-400° C.in 10-20 seconds). This converts the austenitic layer into a hard,metastable martensitic phase with a Rockwell hardness of R_(c)=50-60. Anoptional tempering step can follow the quench cycle, which can furtherimprove the metallurgical properties.

Induction hardened steel parts are designed to have a case hardenedlayer with a specific desired depth. For example, a 25 mm diameter 1050M steel automobile axle may be designed with a hardened layer from 4-5mm thick, as defined by a Rockwell hardness of at least R_(c)=50. Shouldthe layer be too thin, the axle would wear too quickly or haveinsufficient strength; should the layer be too thick, the axle would betoo brittle. During mass production, the measured case depth should berepeatable to within +/−0.1 mm. This requires close control of theinduction heating process, as well as tight control of materialproperties, chemistry, workpiece alignment, etc.

Closed-loop control of the induction heating and hardening processes hasbeen an elusive goal of the industry for many years. Existing inductionhardening equipment is typically operated with open-loop processcontrollers, wherein an operator manually selects power and time (e.g.heating duration). Production users of this equipment monitor theprocess by destructively sectioning finished parts and inspecting theresults; i.e., a finished part is cut apart and the case depth isdirectly measured radially across the cross-section by using a Rockwellhardness indentor, metallographic inspection, or chemical analysis ofthe carbon concentration profile. Process development for new parts isaccomplished by time-consuming and expensive trial-and-error; for agiven coil and part design, heating and quenching parameters are varieduntil destructive analysis reveals that the desired hardness profile isbeing produced.

These parameters are then utilized in the production run and thehardened parts are sampled and analyzed at regular intervals for qualitycontrol and assurance. If the tested part is bad, the production runfrom the previously tested good part is sampled to determine where theprocess failed. Production equipment may be taken out of service untilsubsequent parts test satisfactory. Since each test can take a minimumof several minutes by a trained technician, this process is quiteinefficient for mass production. Unfortunately, small variations in thesteel's chemistry and microstructure can produce unacceptably largevariations in the measured case depths, even for nominally acceptablematerial specifications. The cause of these variations is not wellunderstood.

Other sources of variability include improper part positioning (e.g.misalignment relative to the heating coil), defects in the part (e.g.cracks), and damaged or aged heating coils. Low hardness values measuredon a finished part may be caused by: surface decarburization; lowercarbon content than specified; inadequate austenitizing temperature;prior structure; retained austenite (mostly in high-carbon steels); andunsatisfactory quenching.

Eddy current testing is a commonly used non-destructive method in theautomotive manufacturing industry for measuring case depth. Eddy currenttesting can measure case depths in hardened shafts over a range from 0.2mm to 9 mm, with an accuracy of about 0.15 mm RMS error. See AutomotiveApplication of Eddy Current Testing, in Electromagnetic Testing, Vol. 4,2^(nd) ed., Nondestructive Testing Handbook, American Society ofNondestructive Testing, Inc., 1986, p 424-426. However, to achieve thislevel of accuracy requires the use of a master shaft for calibrationpurposes. This requires destructive measurement of the case depth in themaster shaft by conventional hardness scans. All eddy current responsesfor the batch of test shafts are then normalized to the response for themaster. A computer uses the responses at a few different frequencies toestimate the case depth using multiple linear regression fits. However,this type of eddy-current test is only performed after the part has beeninduction hardened (e.g. on finished parts); it is not used to providereal-time process control.

What is needed is a real-time, non-destructive, non-contact diagnostictechnique that can respond quickly to the temperature changes and phasetransformations in the workpiece during the induction heating process.The diagnostic should be small enough to provide sufficient spatialresolution, and robust enough to withstand the hostile environment (hightemperatures, high fields, large volumes of quenching fluids, etc.). Useof an active feedback of process information measured directly from thepart, coupled with closed-loop control of the heating process, wouldgreatly improve the efficiency of induction hardening systems, whileincreasing accuracy and reducing part rework.

Direct measurement of the workpiece's surface temperature duringinduction heating could provide a useful signal for closed-loop feedbackcontrol. However, use of contact thermocouples is impractical for massproduction, especially since cylindrical parts are often rotated atsignificant rpm's to create uniform heating profiles. Non-contactoptical pyrometry could be used, however the accuracy is affected bysurface conditions (e.g. emissivity) and the operating environment (e.g.smoke, dust, vapors). Coating of the pyrometer's window by the quenchingfluid can also degrade accuracy. Commercially available pyrometers donot have a sufficiently fast response time to monitor the rapid changesin surface temperature during induction heating. Neither pyrometry, norsurface-attached thermocouples, can directly measure the internaltemperatures within a workpiece.

Indirect measurement of the workpiece's temperature, and/or temperatureprofile through the depth, can be inferred by measuring correspondingchanges in the electrical and magnetic properties of the workpiece as itheats up during induction heating. It is well known that the electricalresistivity increases with temperature for typical metals, includingsteel. For example, the resistivity of medium-carbon steels can increaseas much as 800% as the temperature increases from 20° C. to 900° C. SeeASM Handbook, Vol. 4, Heat Treating, 1991, p. 187.

The average electrical resistance of the workpiece (e.g. averaged overthe cross-sectional area) can be measured indirectly by monitoring thevoltage, current, and phase of the induction heating coil. This approachis described in U.S. Pat. No. 5,630,957 (commonly assigned to SandiaCorporation), which is herein incorporated by reference. In this patent,Adkins et al. teach a method of closed-loop control of an inductionhardening machine that uses a trained neural network processor, combinedwith real-time measurement of the voltage, current, and phase in theinduction coil, as measured by a Rogowski coil surrounding a currentlead. The depth of hardening is controlled, in part, by computing theenergy absorbed by the workpiece, and the changes in the averageresistance of the coil plus the workpiece during the heating duration.However, this method does not provide any information regarding thetemperature profile through the depth, or local information at aspecific point on the workpiece.

A non-contacting, miniature magnetic sensor could be used for measuringthe changes in surface magnetic fields near the workpiece in real-timeduring induction heating. A magnetic sensor responds to a time-varyingmagnetic field by generating a time-varying EMF (e.g. voltage) in thesensor's monitor coil. As the workpiece heats up, changes in theelectrical, magnetic, and microstructural properties of the heatedsurface layer affect the surrounding surface magnetic fields. A magneticsensor positioned in close proximity to the surface could detect thesechanges. The output signals from such a sensor could provide usefulinformation for controlling and optimizing the operation of an inductionheating machine.

Magnetic sensors can be divided into two groups: active and passive.Active sensors provide their own excitation fields by using a drivingcoil (e.g. transmission coil or excitation coil). A second sensor coil(e.g. a monitor coil or probe coil responds to the time-varying magneticfield generated by the driving coil, which it is coupled through theworkpiece. The excitation frequency of active sensors could beindependently varied (e.g. 0-10 MHz), and could be much faster than thefrequency of induction heating (e.g. 7 kHz). Use of a variableexcitation frequency could provide the ability to probe the workpiece atvarying depths, since the skin depth is inversely proportional to thesquare root of the driving frequency. Active sensors could also be usedwhen the induction heating coil is temporarily at rest (e.g. duringperiodic pulsed heating), or during the cooling cycle (e.g. duringquenching), when the induction heating coil is turned off. This isbecause active sensors provide their own source of excitation.

Active magnetic sensors can utilize a ferrite core to concentrate themagnetic flux, which improves overall performance. In this case, themonitor coil can sense four components of the total magnetic field: (1)the induction heating field, coupled through the workpiece and theferrite core; (2) the magnetic field produced by eddy currents in theworkpiece, in response to the induction heating field; (3) the highfrequency excitation field, coupled through the ferrite core andworkpiece; and (4) the magnetic field produced by eddy currents inducedin the workpiece by the high frequency excitation field. Generally, thevery small currents induced in the ferrite core can be neglected becauseof its high electrical resistance. Therefore, the only significant eddycurrents are those inside of the workpiece. The magnetic loop includesthe ferrite core and some portion of the workpiece. The magnetic fieldin the ferrite core depends, therefore, on all of the fields generatedinside the workpiece, coupled through the magnetic loop.

Frequency filters could be used to eliminate either the high frequency(e.g. sensor excitation) or the low frequency (e.g. induction heating)components, as well as to control electromagnetic interference (EMI).Also, examination of the phase shift could be used to distinguishbetween these different magnetic components. Additionally, changes inthe orientation of the excitation coil, the monitor coil, the ferritecore, and/or the workpiece could be used to selective emphasize eitherthe coupled applied field, the eddy-current field, or both.

Passive magnetic sensors, on the other hand, do not have an independentexcitation coil. Rather, they respond “passively” to time-varyingchanges in the local surface magnetic field produced by two sources: (1)the magnetic field of the induction heating coil interacting (e.g.coupling) with the workpiece, and (2) the magnetic fields generated bythe induced eddy currents that heat the workpiece. Consequently, thefrequency measured by the passive magnetic sensor is nominally fixed bythe induction heating frequency (e.g. 7 kHz). Despite the fixedfrequency limitation, a passive sensor could be simpler, less expensive,and easier to instrument than an active sensor. Passive sensors couldalso detect the Curie temperature effect (to be discussed later).

Passive magnetic sensors could be used to monitor intra-cycle changes(e.g. during an active heating cycle) in the surface magnetic fieldduring induction heating.

Magnetic sensors could be miniaturized (e.g. 1-2 mm diameter coil), toprovide enhanced spatial and temporal resolution. Additionally, multiplesensors could be placed at various axial locations along an axle ordriveshaft to monitor axial variations in the process. This could beapplied for a continuous hardening process, where the workpiece ismoving sequentially through a fixed set of induction heating coils andquench stations. Alternatively, multiple sensors could be usedavantageously for complicated parts that are being heated simultaneouslywith multiple induction heating coils, each being controlled byindividual controllers coupled to their own sensors.

Although the use of active magnetic sensors have been proposed forcontrolling induction heating machines, numerous problems exist withthese methods. In U.S. Pat. Nos. 5,250,776 and 5,373,143, Pfaffmannteaches a method of using an eddy current sensor to “measure” thetemperature of a part during the induction heating process. The methodrelies on the known increase in electrical resistivity as the workpieceheats up, causing a corresponding change in the impedance of anelectromagnetic test coil placed adjacent to the metal part. SeeIntroduction to Electromagnetic Nondestructive Test Methods, by H. L.Libby, John Wiley & Sons, Inc., 1971, p. 272.

Pfaffmann teaches that because of significant electromagneticinterference (EMI) produced by the induction heating machine, usefulanalysis of the eddy currents sensed by the eddy current sensor isimpaired and, hence, real-time monitoring is not attainable. To getaround this problem, Pfaffmann's method specifically restricts the useof the eddy current sensor to periods of time when the induction heatingcoil is deliberately turned off.

Pfaffmann's method is illustrated in FIG. 1. Here, the induction heatingcoil power is turned off at periodic intervals for short periods of time(e.g. 10 milliseconds). During the period of no heating, the excitationcoil of the eddy current sensor is energized, thereby inducing eddycurrents in the workpiece, which are detected by the sensor coil.Pfaffmann thereby eliminates the problem of electromagnetic interferenceby operating the eddy current sensor only when the induction heatingcoil is deliberately turned off.

Unfortunately, Pfaffmann's method eliminates the possibility of using asimpler and cheaper passive magnetic sensor, since there is noexcitation field to drive the passive sensor when the induction heatingcoil is deliberately turned off. Another disadvantage of Pfaffmann'smethod is that additional electronic equipment is required to create,control, and synchronize the timing of the coordinated patterns forturning on and off the induction heating power, while simultaneouslyactivating the eddy current sensor, thereby adding additional costs andsystem complexity.

Important process information may not be gathered because the largemagnetic field created by the induction heating coil is missing. Forexample, the saturation of the induced magnetic field, B_(sat) (inferromagnetic materials) inside of the heated surface layer isartificially missing when the induction heating coil is deliberatelyturned off. Additionally, commonly used commercial induction heatingmachines that operate on a continuous “harmonic” cycle do not have anatural downtime in the heating cycle. Therefore, costly modificationsof their electronics and control circuitry would be required to createthe downtime period. Pfaffmann does not discuss the important effects ofthe Curie Temperature on the magnetic permeability, which stronglyinfluences the induction heating process (to be discussed later).

In addition to detecting electrical properties, miniature magneticsensors could be used to detect changes in the magnetic properties of aworkpiece, including: (1) hysteresis in the magnetic permeability, and(2) the Curie temperature effect.

Changes in the relative magnetic permeability, mu, (e.g. relative to thefree space permeability) that occur during heating are important tounderstand because the depth of induction heating is inverselyproportional to the square root of the magnetic permeability. For softferromagnetic materials the permeability is a strong non-linear functionof the applied magnetic field. The permeability is defined as the ratioof the Induced Field, B (Teslas), divided by the Applied Field, H (A/m).Above a certain applied field, the induced field saturates at anessentially constant value, B_(sat), which is about 1.5-2 Teslas formedium-carbon steels. This is important because the magnetic fieldapplied by the induction heating coil typically drives the surface ofthe workpiece well beyond magnetic saturation twice during each cycle(both positively and negatively). During saturation, when all of themagnetic domains align with the magnetic field, the induced eddycurrents penetrate more deeply into the part because the permeability,mu, is much smaller inside the saturated zone.

Because alternating current drives the induction heating coil, theworkpiece is subjected to an alternating applied magnetic field. Softferromagnetic materials respond with a hysteresis in their inducedfield, B, when the applied field, H, is cycled between maximum andminimum values, as shown in FIG. 2. Energy lost during AC magnetizationis converted into heat in the ferromagnetic material, and can berepresented, in part, by the area inside the hysteresis loop. The slope(e.g. permeability) of the hysteresis loop, and the flat-top (saturationfield, B_(sat)), both depend on the temperature, driving frequency,carbon content, microstructure, and other properties of the workpiece.FIG. 3 shows an example of how a typical B-H hysteresis loop changeswith temperature for 1050M medium-carbon steel at 5000 Hz. Increasingthe temperature from 100 C to 700 C decreases the saturation field(B_(sat)) by roughly a factor of two. The permeability is also affectedby temperature changes. These changes in magnetic propertiessignificantly affect the heating profile through the depth, and,therefore, the temperature rise during induction heating.

Ferromagnetic materials undergo a dramatic transition from being a“magnetic” material with a large relative magnetic permeability(mu˜100-1000), to being a “non-magnetic” material (mu=1) when heatedabove the Curie temperature. The cause of the Curie effect is closelyrelated to the phase transformation that occurs during heating, passingfrom a 100% ferrite-pearlite BCC microstructure when the temperature isbelow the Ac1 line, to a 100% austenitic FCC microstructure when abovethe A_(c3) line. For steels with carbon concentrations less than about0.45 wt %, the Curie temperature is relatively constant at about 770° C.In higher carbon steels the Curie temperature follows the A₃ line on theFe-C phase diagram to the eutectoid composition; thereafter, itcoincides with the A₁ line.

Both the Curie temperature and the ferrite-to-austenite phasetransformation during heating are affected by many factors, includingthe rate of heating, the starting microstructure (e.g. annealed,normalized, quenched and tempered), grain size, carbon content, traceelemental composition, and possibly magnetic field, frequency, andstress state. When the heated surface exceeds the Curie temperature,austenite begins to form and the magnetic permeability rapidly drops tomu=1. Consequently, the induction heating magnetic field rapidlypenetrates more deeply into the part. Continued heating increases thethickness of the austenite layer, until the desired depth is reached.Finally, the heating coil is turned off and the part is quenched,thereby forming a hard martensite surface layer.

Because the Curie temperature effect coincides closely with thebeginning of austenite formation upon heating, the time after start ofheating at which the Curie temperature is reached at the surface couldbe used as a sensitive indicator of how fast the workpiece is heatingup. For example, if the Curie temperature is reached too quickly, thiscould indicate that too much power is being delivered to the workpiece,resulting in too great a case hardening depth. Likewise, if the Curietemperature signal is detected too late in the process, the case depthcould be too shallow. Either condition could result in rejection of thepart.

The heating process could be adjusted after the Curie temperature hasbeen reached (and detected) by changing the power level of the inductionheating coil, or by adjusting the heating duration (e.g. stop time), sothat the desired depth of case hardening is precisely achieved. Becauseminiature magnetic sensors can detect the Curie temperature effect, theyare well suited to provide critical information useful for activelycontrolling the induction heating process. This applies not only forinduction hardening, but also for high temperature annealing ornormalizing of steel and cast iron parts, using similar inductionheating methods.

Additionally, analysis of the sensor's response throughout the inductionheating process could provide important information regarding thetemperature profile through the depth, and the rate of heating overtime, before the Curie temperature point has been reached. Likewise,similar information could be obtained during cooling when the Curiepoint is traversed when cooling down from a temperature above the Curietemperature point.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (prior art) illustrates the method described in U.S. Pat. No.5,373,143 to Pfaffmann, where the eddy current sensor is activated onlyduring periods of time where the inducting heating coil is turned offand deactivated, which eliminates problems with the eddy current sensordue to electromagnetic interference by the heating coil.

FIG. 2 shows a schematic hysteresis loop for a ferromagnetic material,where “H” is the applied field (A/m), and “B” is the induced field(Tesla).

FIG. 3 shows measured hysteresis loops for a 1050M medium-carbon steel,taken at 5000 Hz, at three different temperatures, 100° C., 400° C., and700° C.

FIG. 4 shows a first schematic example, according to the presentinvention, of a passive magnetic sensor.

FIG. 5 shows a first example, according to the present invention, of theresponse of a passive magnetic sensor during a single cycle of inductionheating, compared to the current in the induction heating coil, asmeasured by a Rogowski coil. The magnetic “spike” can be observed,caused by local saturation of the magnetic properties.

FIG. 6 shows a second example, according to the present invention, ofthe response of a passive magnetic sensor during three pairs cycles ofinduction heating, stacked on top of each other, and taken at threedifferent times during the heating period, 998 ms, 2402 ms, and 3802 msafter the start of heating. The disappearance of the magnetic “spike”can be observed, which corresponds to the Curie temperature point.

FIG. 7 shows a third example, according to the present invention, of theresponse of a passive magnetic sensor during multiple cycles ofinduction heating, stacked on top of each other. The disappearance ofthe magnetic “spike” can be observed, which corresponds to the Curietemperature point.

FIG. 8 shows a fourth example, according to the present invention, ofthe response of a passive magnetic sensor during induction heating. Thesudden shift in the phase angle corresponds to the Curie temperaturepoint.

FIG. 9 shows a first schematic example, according to the presentinvention of a system for providing closed-loop control of the inductionheating process, having a passive magnetic sensor.

FIG. 10 shows a schematic block diagram of a first example of a peakdetector circuit, according to the present invention.

FIG. 11 shows a first schematic example, according to the presentinvention, of an active magnetic sensor.

FIG. 12 shows a first example, according to the present invention, ofthe response of an active magnetic sensor before and after the Curietemperature point has been reached, where the excitation coil has beenlinearly swept over a frequency from 0-10 MHz.

FIG. 13 shows a second schematic example, according to the presentinvention, of a system for providing closed-loop control of theinduction heating process, having an active magnetic sensor.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a method and system of providing real-time,closed-loop control of an induction heating machine by using a miniaturemagnetic sensor to measure the local changes in magnetic field close toa workpiece during induction heating.

FIG. 4 shows a first schematic example of a passive magnetic sensor 8,according to the present invention. The sensing element of passiveinductive sensor 8 is a miniature inductive coil 10. Coil 10 is woundtightly to obtain reasonably high spatial resolution. Coil 10 can bewound on core 12. Core 12 can be a ceramic material to withstand theheat radiated from the surface of the part, and so that eddy currentswould not be induced in the sensor core. The wire can be 30 gauge copperwire, wound on a ceramic rod 12 approximately 1-2 mm in diameter. Thecoil's sensitivity can be increased by using a laminated ferromagneticor ferrite core. However, this can introduce the complication ofunwanted eddy currents in the core of the sensor at certainorientations. The output voltage can be increased by increasing thenumber of turns in coil 10. Coil 10 can have 50 turns, for example. Coilleads 14 can be twisted to minimize pickup of unwanted electromagneticinterference. Coil 10 can be held in close proximity to the surface ofthe workpiece (not shown) with a support member 16. Support member 16can be a hollow tube, made of glass, ceramic, non-magnetic stainlesssteel (e.g. 304, 316 SS) or any non magnetic, high resistivitystructural material that is substantially not heated by the inductionheating process. Twisted pair leads 14 are passed through supportingmember 16 to 50Ω coaxial connectors and cables, which transmit theirsignals to a 50Ω terminated high (e.g. 1 GHz) bandwidth digitaloscilloscope.

Coil 10 and core 12 can be encased in an electrically insulatingmaterial 18. Material 18 can also be used to attach coil 10 and core 12to support member 16. Epoxy was used initially to hold coil 10 in place,but the heat from the inductive hardening process burnt the edges of theepoxy. An inorganic adhesive has been tested which can withstand hightemperatures, but it is slightly soluble in water, and the quenchingliquid may eventually dissolve this adhesive. A preferred material 18 isa water-insoluble, high temperature, insulating adhesive. Athree-dimensional micro-manipulator (not shown) can be used toaccurately position sensor 8 with respect to the workpiece. Sensor 8 canbe mounted to extend through gaps in between sections of the quench head(not shown). The orientation of the coil's major axis, X-X, can beadjusted with respect to the workpiece until the optimum output signalis obtained.

Passive magnetic sensors can respond to at least two components of thetotal surface magnetic field. A first component can be the fieldgenerated by the induction heating coil. A second component can be thefield generated by eddy currents flowing in the workpiece, where theseeddy currents have been induced by the action of the induction heatingcoil. Coil orientation can be chosen to select the applied magneticfield or the magnetic field from the induced eddy currents.

Experiments were performed using two different passive magnetic sensors,and with different orientations with respect to the workpiece. Theworkpiece was a 1 inch diameter by 4 inch long bar of 1040M steel(medium-carbon). The steel bar was heat treated in a single-shotinductive hardening machine located at Sandia National Laboratories inAlbuquerque, N.M. The part was heated for a 5 s period to a surfacetemperature of approximately 1200° C., followed immediately by a 20 squench with a liquid bath. Signals from the two miniature coils, alongwith the signals used to control the machine (e.g. heating voltage andcurrent), were recorded simultaneously with a 1 GHz bandwidthoscilloscope. Many tests were performed to optimize the sensors andevaluate their sensitivity changes with respect to orientation,location, power, and type of material being treated.

The two sensors, designated Coil A and Coil B, were mounted near thesurface of the bar through a gap between the quench heads. The bar wasrotated about its vertical (e.g. long) axis at 12 Hz to improve theuniformity of heating. The axis of coil A was aligned radially(horizontally) from the bar. The axis of coil B was tangential to thesurface of the bar and changed from vertical to horizontal betweentests. Coil A was located directly above coil B by approximately 5 mm.Both coils were initially located near the top of the bar and centeredhorizontally in the gap between the quench heads. At this location theywere most sensitive to the current flowing in the horizontal plane nearthe end of the heating coil. Next, they were moved to the center of thebar vertically, where they were most sensitive to the current which wasflowing in the vertical segments of the coil. In this location, coil Bwas very close to a null in the component of the magnetic field alongits axis, so the sensors were rotated horizontally about the axis of thepart by 5-6 mm. The part was heated with repetitive, nearly sinusoidalpulses (e.g. cycles). The induction heating machine can adjust the“dead” time between the pulses to adjust the power delivered to thepart.

Since the oscilloscope which records the signals did not have enoughmemory to save high resolution waveforms for every pulse, it was set torecord a pair of pulses and then skip 52 pulses before recording anotherpair. In this way, roughly 400 pairs of pulses were saved for all foursets of signals (coil A, coil B, voltage, and current) throughout the 5s heating period. The heating frequency was 5000 Hz. Some of thesewaveforms are shown in FIGS. 5, 6, and 7. In FIGS. 6 and 7, each curveshows two heating cycles. Multiple curves are displayed by shifting themvertically as they increase in time. The starting time is written in thecenter. As can by comparing the start times, the time between curves ismuch longer than the duration of each curve. In other words, manyheating cycles are skipped between curves. The gradual change in themagnetic properties of the surface of the bar as it is heated isindicated by the gradual change in shape and location of thenon-sinusoidal features (e.g. spikes) of these waveforms.

FIG. 5 shows a first example, according to the present invention, of theresponse of a passive magnetic sensor during a single cycle of inductionheating (e.g. the “intra-cycle” response). Curve 20 shows the voltageresponse of the sensor coil. Curve 22 shows the “raw” response of theRogowski coil (dl/dt), which is related to the current flowing in theinduction heating coil. Curve 22 shows a reasonably smooth sinusoidalshape, whereas curve 20 has two distinct spikes 24 in each cycle. Acomparison of these two response curves indicates that the Rogowski coilmeasurements (e.g. curve 22) are not as sensitive as the miniaturemagnetic sensor coil (e.g. curve 20).

The magnetic field, which induces current in the part being heated, onlypenetrates a small distance below the surface called the “skin” depth.This depth is inversely proportional to the square root of threeimportant parameters: (1) the frequency of the alternating magneticfield, (2) the resistivity of the part, and (2) the magneticpermeability of the part. As it heats up, the resistivity of the shellincreases and the magnetic field penetrates deeper.

Magnetic fields in and near magnetic materials exhibit very non-linearbehavior as they oscillate in and out of saturation in positive andnegative directions, as illustrated in FIGS. 2 and 3. As the current isincreased, regions of the material known as magnetic domains align withthe magnetic field. Once all the domains have aligned, increased currentcauses a much slower rate of increase in the magnetic field, as if thematerial had lost its magnetic properties. In an induction heatingmachine, the magnetic fields near the surface go well beyond saturationtwice during each heating cycle. This causes a sudden increase in themagnetic field deeper in the part and a sudden perturbation in the fieldnear the surface of the part. This sudden perturbation is the source ofthe spikes 24 observed by inductive sensor 8 in FIG. 5. The width ofspike 24 in FIG. 5 is only about 2-3 microseconds. Because the transientof spike 24 is so fast, this is one reason why a high bandwidthoscilloscope was used to record the signals.

At high currents or applied field (H), the relative magneticpermeability, mu, goes to 1, so technically the magnetic field (B)continues to vary but with a much smaller modulation while it is on thesaturated portion (ends) of the B-H loop. Saturation occurs at the endsof the B-H loop, where essentially all of the magnetic domains arealigned with the applied field and a further increase in the appliedfield (H) serves to increase the magnetic field (B) directly without theassistance of additional magnetic domains. The applied magnetic fieldpenetrates deeper, because the magnetic permeability has decreased by afactor of about 1000. The skin depth is inversely proportional to thesquare root of (magnetic permeability x conductivity x frequency). Themagnetic field (B) and applied field (H) continue to track the B-H loopduring each cycle. A sinusoidal time-varying applied field produces arectangular time-varying B-field because the B-field continues to changevery rapidly when it crosses zero, but changes very slowly near the endsof the B-H loop. The higher the current, the farther out the linear endsof the loop extend, and the more rectangular is the time-varyingB-field.

A reasonable approximation for the B-H loop can be given by:

B=B ₀tan h((u _(u) −u _(s))(H−/+H _(c)))+H u _(s)

where −/+ indicate the lower/upper branch

u represents magnetic permeability

subscripts s and u represent saturated and unsaturated

H_(c) represents the width of the B-H loop (coercive applied field)

B₀ represents the height of the B-H loop (saturated magnetic field)

FIG. 6 shows a second example, according to the present invention, ofthe response of magnetic sensor 8 during three pairs cycles of inductionheating. The three curves are stacked on top of each other, and weretaken at three different times during the heating period, 998 ms, 2402ms, and 3802 ms after the start of heating. The time derivative of themagnetic field, dB/dt, is plotted. This quantity is proportional to thevoltage induced on the sensor coil A by the time-varying surfacemagnetic field surrounding the workpiece. The numerical integral of thissignal, i.e. the magnetic field, can also be plotted for comparison. Thenon-sinusoidal effects are much more evident in the derivative dB/dt(e.g. “raw” signal) from the sensor coil then in the integrated signal.However, a careful comparison of the integrated signal with a dampedsinusoidal curve also reveals significant non-sinusoidal components. Thedisappearance of the magnetic spike 24 can be readily observed between2402 ms and 3802 ms. Curve 26 (at 3802 ms) shows no evidence of having amagnetic spike 24 (e.g. the response is smooth). As will be explainednext, the disappearance of the magnetic spike 24 is believed tocorrespond to the point in time, t_(curie), when surface temperatureexceeds the Curie temperature.

As the part continues to heat up, the region being saturated movesdeeper. The saturation effects near the surface of the bar cause aslight change in the circuit inductance during the heating cycle, whichproduces a slight shift in frequency or “chirp.” At some point, theoutside of the shell begins to approach the Curie temperature, where thematerial loses its magnetic properties. When the surface temperaturereaches the Curie temperature, the magnetic domains no longer align andthe saturation effect disappears at the surface region. Above the Curietemperature the magnetic permeability decreases by a factor ofapproximately 1000. Consequently, the magnetic field suddenly penetratesmuch deeper. Since the surface is no longer cycling between positive andnegative magnetic saturation, the magnetic spikes 24, disappear. Thiscan be observed in FIG. 6.

FIG. 7 shows a third example, according to the present invention, of theresponse of a passive magnetic sensor during multiple cycles ofinduction heating. Multiple curves are stacked on top of each other, asbefore. The disappearance of the magnetic spike 24 can be readilyobserved, corresponding to the Curie point magnetic transition 28. Inthis example, t_(curie),=2.52 seconds.

The experiments also provided evidence that these coils (e.g. Coils Aand B) can resolve different heating rates and, hence, differenthardening depths at different locations on the part. Because thenon-sinusoidal shape of the sensor response goes through the Curie pointmagnetic transition 28 earlier in the coil located near the top of thebar (plot not shown), we concluded that the bar was being heated fasternear the top. This is confirmed by visual observation of the surfacetemperature, because the bar gets brighter (whiter) near the top than atthe center. It is consistent with the shape of the “hairpin” heatingcoil, which has only vertical components near the center of the bar, buthas vertical and horizontal components near the top.

Data was taken for parts heated with power settings of 600, 700, and 750(arbitrary units). The higher power level causes the transition to occursooner in time, and the first indication of a significant change in themagnetic signal was at 3.3, 2.4, and 2.1 seconds, respectively. This isa clear demonstration that the signals from the sensor coils track theCurie point magnetic transition 28 and can provide the informationnecessary to predict case depth.

FIG. 8 shows a fourth example, according to the present invention, ofthe response of a passive magnetic sensor during induction heating.Curve 30 shows the phase angle, in degrees relative to voltage, ofsensor 8 during the 5 second heating period. In Curve 30, sensor 8 isoriented parallel to the “eddy” fields. Curve 32 shows the phase angleresponse when the sensor 8 is oriented parallel to the applied fields.Curve 36 shows the phase angle response of the Rogowski current monitor.The sudden shift in the phase angle in Curve 30 that occurs at 2.5seconds corresponds to Curie point magnetic transition 28. In thisexample, t_(curie),=2.5 s.

To observe sensor response to a non-magnetic material, measurements weremade on INCONEL 600. The magnetic sensor coil signals looked identicalto the current derivative from the Rogowski monitor. This implies thatthe non-sinusoidal effects (e.g. spikes 24), observed with magneticmaterials are caused predominately by magnetic saturation effects, whichonly occur in magnetic materials (as opposed to eddy currents, whichoccur in both magnetic and non-magnetic conductors).

Data were also taken on an induction heating machine used for annealing,that was located at Chrysler, Inc. Analysis of the data showed magneticeffects that changed in time, even though the material was heated moreslowly than on a typical inductive hardening machine. The axial rotationof the part was easily observed because there was a cut in bar whichtemporarily reduced the magnetic fields at the sensor coil.

The present invention utilizes electronic analog and/or digital filterto minimize the problem of electromagnetic interference produced by theinduction heating coil. Proper probe orientation is also importantbecause good alignment of the long axis of the sensor coil 10, e.g. axisX-X, parallel to the lines of magnetic flux to be measured, can maximizethe sensor's output signal. Conversely, the coil's axis X-X can bealigned perpendicular to the lines of magnetic flux to minimize theoutput signal. In this way, various types of magnetic fields can befiltered out by proper probe orientation. One example would beminimizing the magnetic field due to the induction heating coil, whilemaximizing the field generated by eddy currents in the workpiece, simplyby aligning the coil's X-X axis parallel to the eddy current inducedfield, and perpendicular to the heating coil field. As such, the presentinvention can operate the magnetic sensors during the time when theinduction heating coil is energized. It is not necessary to turn theheating coil off. This is an especially useful advantage for FET-basedinductive heating machines that provide continuously pulsed,near-sinusoidal operation, without any quiet or “dead” time.

A first example, according to the present invention, of a method ofdetecting the time, t_(curie), as measured from the start of inductionheating, when the surface temperature of a ferromagnetic workpiece hasexceeded the Curie temperature during induction heating, can include thefollowing steps: providing a ferromagnetic workpiece having a regionthat is being heated by an induction heating coil, the workpiece havinga time-varying surface magnetic field; placing a magnetic sensor (eitherpassive or active) in close proximity to the heated region; measuringthe sensor's response to the time-varying surface magnetic field, whilethe workpiece is being inductively heated; calculating the time rate ofchange of the surface magnetic field, dB/dt, as a function of time, fromthe sensor's output; plotting the intra-cycle variation in dB/dt versustime for at least one cycle of induction heating; identifying acharacteristic spike 24 in the plot of dB/dt versus time, caused by aperturbation in the surface magnetic field due to saturation of themagnetic permeability; identifying when spike 24 disappears, due to theloss of magnetic properties when the workpiece surface temperatureexceeds the Curie temperature (e.g. Curie point 28); and subsequentlyidentifying the time, t_(curie), when spike 24 disappearance occurs.

A second example, according to the present invention, of a method ofdetecting the time, t_(curie), as measured from the start of inductionheating, when the surface temperature of a ferromagnetic workpiece hasexceeded the Curie temperature during induction heating, can include thefollowing steps: providing a ferromagnetic workpiece having a regionthat is being heated by an induction heating coil, the workpiece havinga time-varying surface magnetic field; placing a magnetic sensor (eitherpassive or active) in close proximity to the heated region; measuringthe sensor's response to the time-varying surface magnetic field, whilethe workpiece is being inductively heated; calculating the phase angleof the sensor's output relative to the phase of the applied inductionheating field; plotting the intra-cycle phase angle versus time, overthe period of induction heating; identifying a characteristic shift 28in the plot of phase angle versus time, due to the loss of magneticproperties when the workpiece surface temperature exceeds the Curietemperature; and subsequently identifying the time, t_(curie), when thecharacteristic shift 28 occurs.

Use of an active feedback of process information measured directly fromthe workpiece, coupled with closed-loop control of the heating process,can greatly improve the efficiency of induction hardening systems, whileincreasing accuracy and reducing part rework.

FIG. 9 shows a first schematic example, according to the presentinvention, of a system for providing closed-loop control of theinduction heating process, having a passive magnetic sensor. Workpiece36 is shown partially surrounded by an inductive heating coil 38 havinga hairpin-type geometry. Workpiece 36 can rotate about its longitudinalaxis in order to improve the uniformity of heating. Rogowski coil 40surrounds a current lead to heating coil 38. Rogowski coils are wellknown in the art, and respond to time-varying changes in the currentpassing through the coil (e.g. the derivative of the current withrespect to time, dl/dt). The signal can then be integrated to provideactual current. Induction heating coil power supply 42 providesalternating current to heating coil 38. The driving frequency of the ACcurrent to heating coil 38 can be 5-10 kHz, depending on theapplication. Passive magnetic sensor probe 8, with attached sensor coil10, is positioned in close proximity to workpiece 36. Output signalsfrom passive sensor 8 pass through electronic filter 44, which filtersout unwanted electromagnetic interference. These signals are measuredand recorded by a voltage and/or current monitor 46, (e.g. a 1 GHzbandwidth oscilloscope). Signal processor 48 processes the measuredvoltage and current signals. Signal processor 48 can be amicroprocessor, digital signal processor chip, or an analog circuit, anyof which is designed to perform simple, high speed math function on datasignals. Signal processor 48 can calculate the phase angle of thesignals. Additionally, signal processor 48 can compare the measuredsignals with preprogrammed waveforms to create an error signal that canbe used to provide feedback control information to power supply 42 viacontrol signal 50.

Alternatively, signal processor 48 can be used to calculate one or moreattributes that can be derived from the waveforms of the measured sensorsignals. Examples of suitable attributes include a measure of thedeviation from non-sinusoidal behavior of the waveforms. Forferromagnetic materials, a useful attribute would be a calculation ofthe time, t_(curie), when the Curie temperature point has been reached.As discussed above, t_(curie), can be calculated in a number ofdifferent ways, including identifying the time when the magnetic spikes24 disappear, or by identifying the characteristic shift in the plot ofthe intra-cycle phase angle versus time (e.g. curve 30 in FIG. 8), for asensor 8 properly aligned to maximize the response to the induced eddycurrents fields.

Signal processor 48, or analog electronics circuits, can compare the oneor more calculated attributes of the measured sensor signals to one ormore desired attributes to create an error signal. For example, theerror signal can be the difference in time between the measured Curietemperature point, t_(curie), and the desired time when the Curie pointshould have been reached. This difference in time can be used to createa control signal 50 that feeds back to the induction heating powersupply 42. Feedback control signal 50 can be used to adjust the powerlevel of the induction heating power supply 42, or the shut-off time,t_(off), of the current used to drive the induction heating coil 38. Thecontrol signal can include a proportional adjustment in the operatingparameter (e.g. power level or shut-off time). For example, if themeasured Curie temperature point, t_(curie), occurred at a time 10%longer than the desired time, then the power level or shut-off timecould be increased by 10% to correct for the delayed Curie pointresponse. Alternatively, signal processor 48 can utilize a moresophisticated algorithm for determining the correct amount ofadjustment, which can be based on complex models for the workpiece'scoupled thermal and electromagnetic behavior. Alternatively, thealgorithm used by signal processor 48 can be a neural network programthat has been previously trained with data taken from previous heatingruns (as described by Adkins et al. in U.S. Pat. No. 5,630,957). Thegoal of making adjustments to power supply 42 is to reduce the magnitudeof the error signal below a predetermined acceptable limit.

Control signal 50 can also be used to adjust the relative position ofworkpiece 36 with respect to heating coil 38. This could be used for aworkpiece that is being scanned (not shown) through a fixed heating coil38.

The steps of measuring the sensors response, creating an error signal,and reducing the error signal by adjusting the machine's operation canbe repeated as many times as needed during the induction heating period,in order to achieve the required parameters.

A specially designed electronic circuit 47 has been designed and used toautomatically detect the disappearance of the magnetic spikes 24 at theCurie point magnetic transition 28. This peak detector circuit 47comprises a band-pass filter, followed by a peak amplitude detector. Thefilter eliminates the lower frequency (e.g. 7 kHz) heating signal, andleaves only the magnetic spikes. The peak detector 47 produces ademodulated signal that is a measure of the changes in the B-H loop.Below the Curie temperature, this circuit has a large signal that goesessentially to zero when the Curie temperature is reached.

FIG. 10 shows a schematic block diagram of a first example of a peakdetector circuit 47, according to the present invention. One or moresignals 46 from the magnetic sensor are fed into circuit 47. The firstelement in the peak detector circuit can be a band-pass filter 72.Band-pass filter 72 can be, for example, a fourth-order system with acenter frequency of 250 kHz, a bandwidth of 74 kHz, and a damping ratioof 0.3. Band-pass filter 72 can be designed to reject the low-frequencyinduction heating signal, as well as higher-frequency measurement noise,while passing through the desired magnetic current spikes. Because thespikes have short rise-times, it is not practical to use digitalsampling and filtering techniques to process the signals (requiringgigahertz sample rates and gigabytes of storage). therefore, theband-pass filter 72 is preferably implemented with high-speedoperational amplifiers using a two-stage Kerwin-Huelsman-Newcomb design.

The output of the band-pass filter 72 can be rectified using a precisionanalog full-wave rectifier circuit 74. Once rectified, the signal can befed into the amplitude peak detector 76. Finally the output of the peakdetector 76 can be processed by low-pass filter 78 to yield a lowfrequency signal that is proportional to the amplitude of the magneticspikes. High-band width amplifiers with a minimum slew rate of 20Volts/microsecond can be used for the first three stages of the peakdetector circuit 47, while the low-pass filter section 78 can work withany standard operational amplifier. The output of peak detector circuit47 goes to signal processor 48 for further analysis and/or forclosed-loop process control.

Active magnetic sensors can also be used with the method and systemdescribed above for closed-loop control of the induction heatingprocess. FIG. 11 shows a first schematic example, according to thepresent invention, of an active magnetic sensor 52. Sensor 52 comprisesat least one miniature excitation coil 58 and at least one miniaturemonitor (e.g. sensor) coil 56. The coil windings can be wound around atoroidally-shaped ferrite core 54. The ferrite core 54 has lowelectrical conductivity, which minimizes the unwanted effects of inducededdy currents in core 54. The toroidal shape of the core 54, plus themagnetic material inside of core 54, can concentrate the magnetic fluxlines 60 to penetrate the workpiece 36 in the near-surface region andconnect back to the monitor coil 56. Other shapes can be used for aferrite core 54, such as a linear core, or cylindrical core, as is wellknown in the art.

Monitor coil 56 responds primarily to the magnetic field generated bythe excitation coil 58. A primary advantage of using an active magneticsensor, compared to a passive magnetic sensor, is that the frequency ofthe active sensor (i.e. excitation coil 58) can be varied independentlyfrom the driving frequency of the induction heating coil 38 (which istypically fixed at around 5-10 kHz, but can be lower or higher,depending on the application). For example, the driving frequency of theexcitation coil 58 can be swept from 0-10 MHz to change the depth ofsensitivity, and to induce a wide range of response from the monitorcoil 56. Alternatively, a broadband impulse “burst” can be used to driveexcitation coil 58, which also provides a wide range of drivingfrequencies.

Active magnetic sensors can respond to at least four components of thetotal coupled magnetic fields. A first component can be the fieldgenerated by the induction heating coil 38. A second component can bethe field generated by a first set of eddy currents flowing in theworkpiece, where this first set of eddy currents have been induced bythe action of the induction heating coil 38. A third component can bethe field generated by the active sensor's excitation coil 58. A fourthcomponent can be the field generated by a second set of eddy currentsflowing in the workpiece, where this second set of eddy currents havebeen induced by the action of the active sensor's excitation coil 58.

FIG. 12 shows a first example, according to the present invention, ofthe response of an active magnetic sensor before and after the Curietemperature point has been reached, where the excitation coil has beenlinearly swept over a frequency from 0-10 MHz. Curve 61 shows theexcitation frequency. Curve 62 shows the characteristic response of themonitor coil 56 (in Volts), as a function of the driving frequency from0-10 MHz. A broad resonance can be observed at omega-1. At a later time,after the Curie temperature point has been reached, Curve 64 displays anumber of altered features. In Curve 64, the resonance frequency hasshifted to a higher value, omega-2. Also, the amplitude of the resonancehas increased, as well as the “Q” value. Any three of thesecharacteristic features of the response curve, or combinations of them,can be used as the attribute in the closed-loop control program tocreate the error signal used to provide the feedback control signal 50to power supply 42.

FIG. 13 shows a second schematic example, according to the presentinvention, of a system for providing closed-loop control of theinduction heating process, having an active magnetic sensor. The systemis identical to that shown in FIG. 9, except that the passive magneticsensor 8 has been replaced with an active magnetic sensor 52. Activesensor 52 has an excitation coil 58, which is driven by an excitationcoil driver 66. Sensor 52 can have a toroidally-shaped core 54. Driver66 can sweep the frequency of excitation coil 58 from 0-10 MHz. Filter44 can reduce electromagnetic interference from induction heating coil38, thereby allowing active sensor 52 to be used during operation of aninduction heating cycle.

When the methods and systems described above are applied to inductionhardening machines, the error signal provides a useful feedback controlto adjust, in real-time, the depth of case hardening, towards thedesired value.

The particular sizes and equipment discussed above are cited merely toillustrate a particular embodiment of this invention. It is contemplatedthat the use of the invention may involve components or methods havingdifferent characteristics. For example, the method and system describedabove can be used to provide monitoring and/or closed loop control of aquenching machine. In this case, for example, with a ferromagneticworkpiece that has some portion already heated above the Curietemperature point 28, the present invention can be used to detect thepoint in time during cooling (e.g. quenching) when the sensors indicatea transition from being above the Curie temperature to being below theCurie temperature. This could manifest itself as the reappearance ofspikes 24 in the intra-cycle curves of the passive sensor response, oras a characteristic shift in the phase angle. Or, with active sensors,as characteristic shifts in the resonance, amplitude, or “Q” value.

It is intended that the scope of the invention be defined by the claimsappended hereto.

We claim:
 1. A closed-loop method for controlling the operation of ainduction heating machine, comprising: a) placing a workpiece in closeproximity to an induction heating coil; b) heating a region of theworkpiece with the induction heating coil, wherein the workpiece has atime-varying total surface magnetic field comprising a primary magneticfield component generated by the induction heating coil, and a secondarymagnetic field component generated by eddy currents induced in theworkpiece by the induction heating coil; c) positioning a magneticsensor outside of the workpiece in close proximity to the outer surfaceof the workpiece's heated region; d) measuring the sensor's response tothe time-varying total surface magnetic field, while the workpiece isbeing inductively heated; e) comparing the measured sensor response to adesired sensor response; f) adjusting in real-time the operation of theinduction heating machine in a manner so as to reduce the differencebetween the measured sensor response and the desired sensor response;and g) repeating steps d), e), and f) as many times as is necessaryuntil the induction heating process has been completed.
 2. The method ofclaim 1, wherein the sensor's response comprises a parameter selectedfrom the group consisting of a time-varying voltage, a time-varyingcurrent, and a time-varying phase angle between the voltage and current,or any combination thereof.
 3. The method of claim 1, wherein themagnetic sensor comprises a miniature coil having a diameter less than 3mm; and a water-insoluble, electrically insulating coating surroundingthe coil's windings.
 4. The method of claim 3, wherein positioning themagnetic sensor in step c) further comprises: h) aligning the miniaturecoil's axis to be perpendicular to the induction heating magnetic field,thereby minimizing the sensor's output signal responsive to the primarymagnetic field component generated by the induction heating coil; and i)aligning the miniature coil's axis to be parallel to the eddy currentinduced magnetic field, thereby maximizing the sensor's output signalresponsive to the secondary magnetic field component generated by eddycurrents induced in the workpiece by the induction heating coil.
 5. Themethod of claim 1, wherein adjusting the operation of the inductionheating machine in step f) comprises adjusting the shut-off time forturning the induction heating coil off.
 6. The method of claim 1,wherein adjusting the operation of the induction heating machine in stepf) comprises adjusting the power level of the induction heating machineduring induction heating.
 7. The method of claim 1, wherein adjustingthe operation of the induction heating machine in step f) comprisesadjusting the position of the workpiece's heated region relative to theposition of the induction heating coil during induction heating.
 8. Themethod of claim 1, wherein adjusting the operation of the inductionheating machine in step f) comprises using a signal processor to processthe sensor's response; wherein the signal processor uses a neuralnetwork program that has been previously trained with data taken fromprevious heating runs.
 9. The method of claim 1, further comprisingelectronically filtering out electromagnetic interference from thesensor's output signal.
 10. The method of claim 1, further comprisingorienting the position of the sensor so as to maximize the currentinduced in the sensor by eddy currents flowing within the workpiece. 11.The method of claim 1, wherein the magnetic sensor comprises a passivemagnetic sensor; and wherein the method further comprises: h)identifying a distinct magnetic spike in the passive sensor's outputduring an induction heating cycle that is caused by a suddenperturbation in the total surface magnetic field, due to localsaturation of the magnetic permeability of the workpiece that is createdwhen the saturation magnetic field, B_(sat), of the workpiece has beenexceeded.
 12. The method of claim 11, wherein the workpiece comprises aferromagnetic material, and wherein the method further comprises: i)calculating the time rate of change of the total surface magnetic field,dB/dt, as a function of time, from the sensor's output; j) plotting theintra-cycle variation in dB/dt versus time for at least one cycle ofinduction heating; k) identifying a characteristic spike in the plot ofdB/dt versus time, caused by a perturbation in the surface magneticfield due to saturation of the magnetic permeability; l) identifying thedisappearance of the characteristic spike, which is caused by the lossof magnetic properties when the workpiece's surface temperature exceedsthe Curie Temperature of the ferromagnetic material; m) determining themeasured Curie Time, t_(curie), when the disappearance of thecharacteristic spike occurs in step k); and n) shutting-off theinduction heating coil when the measured Curie Time, t_(curie), is equalto or greater than a desired Curie Time.
 13. The method of claim 12,wherein identifying the disappearance of the characteristic spike instep l) comprises: o) filtering the sensor's output signal through aband-pass filter, thereby rejecting any low-frequency induction heatingsignal and rejecting any higher-frequency measurement noise, whileallowing the signal of the characteristic spike to pass through; p)rectifying the output of the band-pass filter from step o) using aprecision analog full-wave rectifier circuit; q) feeding the rectifiedsignal from step p) into an amplitude peak detector module; r) filteringthe output of the amplitude peak detector module from step q) through alow-pass filter to yield a low frequency signal that is proportional tothe amplitude of the distinct magnetic spike's signal; and s) processingthe low frequency signal from step r) with a signal processor, therebydetecting the disappearance of the characteristic spike caused when theCurie temperature of the workpiece is exceeded.
 14. The method of claim11, wherein the step of identifying the characteristic spike in thesensor's output during step h) comprises using a high bandwidthoscilloscope to record the sensor's output signal.
 15. The method ofclaim 1, wherein the workpiece comprises a ferromagnetic material, andwherein the method further comprises: h) calculating the phase angle ofthe sensor's output signal relative to the phase of the current flowingin the induction heating coil; i) plotting the phase angle versus time;j) identifying a characteristic shift in the plot of phase angle versustime, which is caused by the loss of magnetic properties when theworkpiece surface temperature exceeds the Curie temperature of theferromagnetic material; and k) determining the measured Curie Time,t_(curie), when the characteristic shift occurs in step j); and l)shutting-off the induction heating coil when the measured Curie Time,t_(curie), is equal to or greater than a desired Curie Time.
 16. Themethod of claim 1, wherein the magnetic sensor comprises an activemagnetic sensor; wherein the active magnetic sensor comprises aminiature excitation coil and a monitor coil; and wherein the methodfurther comprises: h) generating an excitation magnetic field byenergizing the miniature excitation coil; and i) measuring the responseof the monitor coil to the total magnetic field generated by both theinduction heating coil and the excitation coil.
 17. The method of claim16, wherein energizing the miniature excitation coil comprises using abroadband impulse burst to drive the miniature excitation coil, wherebya wide range of driving frequencies are produced.
 18. The method ofclaim 1, further comprising, during induction heating: h) sweeping thedriving frequency of the excitation coil from 0-10 MHz; i) generating afirst output spectrum by measuring the monitor coil's output over theswept frequency range of 0-10 MHz; and j) identifying a first resonancein the first output spectrum; then k) repeating steps h) through j),thereby generating a second output spectrum; l) indentifying a secondresonance in the second output spectrum; m) comparing the first andsecond output spectra; n) indentifying a characteristic change betweenthe first and second output spectra that is caused when the Curietemperature of the workpiece is exceeded during induction heating; o)comparing the measured characteristic change to a desired characteristicchange; p) adjusting in real-time the operation of the induction heatingmachine in a manner so as to reduce the difference between the measuredcharacteristic change and the desired characteristic change; and q)repeating steps h) through p) many times as is necessary until theinduction heating process has been completed.
 19. The method of claim18, wherein the characteristic change between the first and secondoutput spectra comprises an attribute selected from the group consistingof a shift in the output spectrum's resonance frequency, a change in theamplitude of the output spectrum's resonance, and a shift in the“Q”-value of the output spectrum.
 20. A system for providing closed-loopcontrol of the operation of a induction heating machine used forinduction heating a region of a workpiece, comprising: a magnetic sensorpositioned outside of the workpiece in close proximity to the outersurface of the workpiece's heated region; means for measuring thesensor's response to a time-varying total surface magnetic fieldgenerated during induction heating; means for comparing the measuredsensor response to a desired sensor response; means for adjusting inreal-time the operation of the induction heating machine in a manner soas to reduce the difference between the measured sensor response and thedesired sensor response; and means for repeatedly using the measuringmeans, the comparing means, and the adjusting means to provideclosed-loop control of the induction heating process.
 21. The system ofclaim 20, wherein the magnetic sensor comprises a miniature coil havinga diameter less than 3 mm; and a water-insoluble, electricallyinsulating coating surrounding the coil's windings.
 22. The system ofclaim 21, wherein the magnetic sensor comprises 30 gauge copper wirewound at least 50 times around a ceramic rod having a diameter of about1-2 mm.
 23. The system of claim 20, further comprising signal processingmeans for processing the measured signals from the magnetic sensor,comparing the measured signals with preprogrammed waveforms to create anerror signal, and using that error signal to provide feedback controlinformation to the induction heating machine's power supply via acontrol signal.
 24. The system of claim 23, wherein the signalprocessing means comprises a neural network program that has beenpreviously trained with data taken from previous heating runs.
 25. Thesystem of claim 20, wherein the means for measuring the sensor'sresponse comprises a high bandwidth oscilloscope for recording thesensor's output signal.
 26. The system of claim 20, further comprisingmeans for filtering out electromagnetic interference from the sensor'soutput signal disposed in-between the magnetic sensor and the measuringmeans.
 27. The system of claim 20, wherein the magnetic sensor comprisesa passive magnetic sensor, and wherein the system further comprises apeak detector circuit for detecting the disappearance of characteristicmagnetic spikes caused by the loss of magnetic properties when thesurface temperature of a workpiece comprising ferromagnetic materialexceeds the Curie temperature of the ferromagnetic material.
 28. Thesystem of claim 27, wherein the peak detector circuit further compriseat least one high-band width amplifier with a minimum slew rate of 20Volts/microsecond.
 29. The system of claim 27, wherein the peak detectorcircuit comprises a band-pass filter operatively coupled to an amplitudepeak detector module, for producing a demodulated signal that is ameasure of changes in the B-H loop.
 30. The system of claim 29, whereinthe peak detector circuit further comprises: a precision analogfull-wave rectifier circuit operatively coupled to the band-pass filter;and a low-pass filter operatively coupled to the full-wave rectifiercircuit.
 31. The system of claim 29, wherein the band-pass filter isimplemented with high-speed operational amplifiers using a two-stageKervin-Huelsman-Newcomb design.
 32. The method of claim 29, wherein theband-pass filter comprises a fourth-order system with a center frequencyof 250 KHz, a bandwidth of about 74 KHz, and a damping ratio of 0.3. 33.The system of claim 20, wherein the magnetic sensor comprises a passivemagnetic sensor, and wherein the system further comprises: means forcalculating the phase angle of the sensor's output signal relative tothe phase of the current flowing in the induction heating coil; meansfor plotting the phase angle versus time; means for identifying acharacteristic shift in the plot of phase angle versus time, which iscaused by the loss of magnetic properties when the workpiece surfacetemperature exceeds the Curie temperature of the ferromagnetic material;and means for determining the measured Curie Time, t_(curie), when saidcharacteristic shift occurs.
 34. The system of claim 20, wherein themagnetic sensor is mounted in a gap disposed between two adjacentsections of a quench head used to quench the heated workpiece afterinduction heating.
 35. The system of claim 20, wherein the position ofthe magnetic sensor is oriented so as to maximize the current induced inthe sensor by eddy currents flowing within the workpiece.
 36. The systemof claim 20, wherein the induction heating machine comprises FET-basedinductive heating machine that provides continuously pulsed,near-sinusoidal operation without any dead time between heating cycles.37. The system of claim 20, wherein the magnetic sensor comprises anactive sensor, comprising an excitation coil and a monitor coil.
 38. Thesystem of claim 37, further comprising a toroidally-shaped ferrite coredisposed inside the excitation coil and monitor coil of the activemagnetic sensor.
 39. The system of claim 37, further comprising meansfor sweeping the driving frequency of the excitation coil from 0-10 MHz.40. The system of claim 37, further comprising means for driving theexcitation coil with a broadband impulse burst, thereby providing a widerange of driving frequencies.
 41. The system of claim 20, wherein themeans for adjusting the operation of the induction heating machinecomprises means selected from the group consisting of means foradjusting the shut-off time for turning the induction heating coil off,means for adjusting the power level of the induction heating machineduring induction heating, and means for adjusting the position of theworkpiece's heated region relative to the position of the inductionheating coil during induction heating.
 42. A system for providingclosed-loop control of the operation of a induction heating machine usedfor induction heating a region of a workpiece, comprising: a magneticsensor positioned outside of the workpiece in close proximity to theouter surface of the workpiece's heated region; means for measuring thesensor's response to a time-varying total surface magnetic fieldgenerated during induction heating; means for comparing the measuredsensor response to a desired sensor response; means for adjusting inreal-time the operation of the induction heating machine in a manner soas to reduce the difference between the measured sensor response and thedesired sensor response; and means for repeatedly using the measuringmeans, the comparing means, and the adjusting means to provideclosed-loop control of the induction heating process; wherein themagnetic sensor comprises a passive magnetic sensor; and wherein thesystem further comprises a peak detector circuit for detecting thedisappearance of characteristic magnetic spikes caused by the loss ofmagnetic properties when the surface temperature of a workpiececomprising ferromagnetic material exceeds the Curie temperature of theferromagnetic material.